Method for fabricating a power semiconductor device having a voltage sustaining layer with a terraced trench facilitating formation of floating islands

ABSTRACT

A method is provided for forming a power semiconductor device. The method begins by providing a substrate of a first conductivity type and then forming a voltage sustaining region on the substrate. The voltage sustaining region is formed by depositing an epitaxial layer of a first conductivity type on the substrate and forming at least one terraced trench in the epitaxial layer. The terraced trench has a plurality of portions that differ in width to define at least one annular ledge therebetween. A barrier material is deposited along the walls of the trench. A dopant of a second conductivity type is implanted through the barrier material lining the annular ledge and said trench bottom and into adjacent portions of the epitaxial layer. The dopant is diffused to form at least one annular doped region in the epitaxial layer and at least one other region located below the annular doped region. A filler material is deposited in the terraced trench to substantially fill the trench, thus completing the voltage sustaining region. At least one region of the second conductivity type is formed over the voltage sustaining region to define a junction therebetween.

RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No.09/970,758 entitled “Method For Fabricating A Power Semiconductor DeviceHaving A Voltage Sustaining Layer With A Terraced Trench FacilitatingFormation Of Floating Islands” filed on Oct. 4, 2001 now U.S. Pat. No.6,649,477.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor power devices,and more particularly to a semiconductor power device such as a MOSFETand other power devices that use floating islands of oppositely dopedmaterial to form the voltage sustaining layer.

BACKGROUND OF THE INVENTION

Semiconductor power devices such as vertical DMOS, V-groove DMOS, andtrench DMOS MOSFETs, IGBTs as well as diodes and bipolar transistors areemployed in applications such as automobile electrical systems, powersupplies, motor drives, and other power control applications. Suchdevices are required to sustain high voltage in the off-state whilehaving low on-resistance or a low voltage drop with high current densityin the on-state.

FIG. 1 illustrates a typical structure for an N-channel power MOSFET. AnN-epitaxial silicon layer 101 formed over an N+ doped silicon substrate102 contains p-body regions 105 a and 106 a, and N+ source regions 107and 108 for two MOSFET cells in the device. P-body regions 105 and 106may also include deep p-body regions 105 b and 106 b. A source-bodyelectrode 112 extends across certain surface portions of epitaxial layer101 to contact the source and body regions. The N-type drain for bothcells is formed by the portion of N-type epitaxial layer 101 extendingto the upper semiconductor surface in FIG. 1. A drain electrode isprovided at the bottom of N+ doped substrate 102. An insulated gateelectrode 118 comprising insulating and conducting layers, e.g., oxideand polysilicon layers, lies over the body where the channel will beformed and over drain portions of the epitaxial layer.

The on-resistance of the conventional MOSFET shown in FIG. 1 isdetermined largely by the drift zone resistance in epitaxial layer 101.Epitaxial layer 101 is also sometimes referred to as a voltagesustaining layer since the reverse voltage applied between the N+ dopedsubstrate and the P+ doped deep body regions is sustained by epitaxiallayer 101. The drift zone resistance is in turn determined by the dopingconcentration and the thickness of epitaxial layer 101. However, toincrease the breakdown voltage of the device, the doping concentrationof epitaxial layer 101 must be reduced while the layer thickness isincreased. The curve in FIG. 2 shows the on-resistance per unit area asa function of the breakdown voltage for a conventional MOSFET.Unfortunately, as the curve shows, the on-resistance of the deviceincreases rapidly as its breakdown voltage increases. This rapidincrease in resistance presents a problem when the MOSFET is to beoperated at higher voltages, particularly at voltages greater than a fewhundred volts.

FIG. 3 shows a MOSFET that is designed to operate at higher voltageswith a reduced on-resistance. This MOSFET is disclosed in Cezac et al.,Proceedings of the ISPSD, May 2000, pp. 69-72, and Chen et al., IEEETransactions on Electron Devices, Vol. 47, No. 6, June 2000, pp.1280-1285, which are hereby incorporated by reference in their entirety.This MOSFET is similar to the conventional MOSFET shown in FIG. 1 exceptthat it includes a series of vertically separated P− doped layers 310 ₁,310 ₂, 310 ₃, . . . 310 _(n) (so-called “floating islands”), which arelocated in the drift region of the voltage sustaining layer 301. Thefloating islands 310 ₁, 310 ₂, 310 ₃, . . . 310 _(n) produce an electricfield that is lower than for a structure with no floating islands. Thelower electric field allows a higher dopant concentration to be used inthe epitaxial layer that in part, forms the voltage sustaining layer301. The floating islands produce a saw-shaped electric field profile,the integral of which leads to a sustained voltage obtained with ahigher dopant concentration than the concentration used in conventionaldevices. This higher dopant concentration, in turn, produces a devicehaving an on-resistance that is lower than that of a device without oneor more layers of floating islands.

The structure shown in FIG. 3 can be fabricated with a process sequencethat includes multiple epitaxial deposition steps, each followed by theintroduction of the appropriate dopant. Unfortunately, epitaxialdeposition steps are expensive to perform and thus a structure that usesmultiple epitaxial deposition steps is expensive to manufacture.

Accordingly, it would be desirable to provide a method of fabricating apower semiconductor device such as the MOSFET structure shown in FIG. 3,which method requires a minimum number of epitaxial deposition steps sothat the device can be produced less expensively.

SUMMARY OF THE INVENTION

In accordance with the present invention, a method is provided forforming a power semiconductor device. The method begins by providing asubstrate of a first conductivity type and then forming a voltagesustaining region on the substrate. The voltage sustaining region isformed by depositing an epitaxial layer of a first conductivity type onthe substrate and forming at least one terraced trench in the epitaxiallayer. The terraced trench has a plurality of portions that differ inwidth to define at least one annular ledge therebetween. A barriermaterial is deposited along the walls of the trench. A dopant of asecond conductivity type is implanted through the barrier materiallining the annular ledge and said trench bottom and into adjacentportions of the epitaxial layer. The dopant is diffused to form at leastone annular doped region in the epitaxial layer. One other regionlocated below the annular doped region may also be formed. A fillermaterial is deposited in the terraced trench to substantially fill thetrench, thus completing the voltage sustaining region. At least oneregion of the second conductivity type is formed over the voltagesustaining region to define a junction therebetween.

The power semiconductor device formed by the inventive method may beselected from the group consisting of a vertical DMOS, V-groove DMOS,and a trench DMOS MOSFET, an IGBT, a bipolar transistor, and diodes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross-sectional view of a conventional power MOSFETstructure.

FIG. 2 shows the on-resistance per unit area as a function of thebreakdown voltage for a conventional power MOSFET.

FIG. 3 shows a MOSFET structure that includes a voltage sustainingregion with floating islands located below the body region, which isdesigned to operate with a lower on-resistance per unit area at the samevoltage than the structure depicted in FIG. 1.

FIG. 4 shows a MOSFET structure that includes a voltage sustainingregion with floating islands both below and between the body regions.

FIGS. 5( a)-5(g) show a sequence of exemplary process steps that may beemployed to fabricate a voltage sustaining region constructed inaccordance with the present invention.

DETAILED DESCRIPTION

FIG. 4 shows a power semiconductor device having floating islands of thetype disclosed in co-pending U.S. Application Ser. No. 09/970,972. Inthis device the trenches are assumed to be circular and therefore thefloating islands are depicted as donut-shaped. Of course, the trenchesmay have other shapes such squares, rectangles, hexagons, or the like,which in turn will determine the shape of the floating islands. AnN-type epitaxial silicon layer 401 formed over an N+ silicon substrate402 contains P-body regions 405, and N+ source regions 407 for twoMOSFET cells in the device. As shown, P-body regions 405 a may alsoinclude deep P-body regions 405 b. A source-body electrode 412 extendsacross certain surface portions of epitaxial layer 401 to contact thesource and body regions. The N-type drain for both cells is formed bythe portion of N-epitaxial layer 401 extending to the uppersemiconductor surface. A drain electrode is provided at the bottom of N+substrate 402. An insulated gate electrode 418 comprising oxide andpolysilicon layers lies over the channel and drain portions of the body.A series of floating islands 410 are located in the voltage sustainingregion of the device defined by epitaxial silicon layer 401. Thefloating islands are arranged in an array when viewed from the top ofthe device. For instance, in FIG. 4, in the “y” direction, floatingislands are denoted by reference numerals 410 ₁₁, 410 ₁₂, 410 ₁₃, . . .410 _(1m) and in the “z” direction floating islands are denoted byreference numerals 410 ₁₁, 410 ₂₁, 410 ₃₁, . . . 410 _(m1). While thecolumn of floating islands 410 located below the gate 418 may or may notbe employed, they are preferably employed when required for the devicegeometry and the resistivity of epitaxial layer 401.

In the device of FIG. 4, each horizontal row of floating islands, suchas row 410 ₁₁, 410 ₁₂, 410 ₁₃, . . . 410 _(1m), is formed in a separateimplantation step. While this fabrication technique advantageouslyreduces the required number of epitaxial deposition steps in comparisonto the known fabrication technique discussed in connection with FIG. 3,it would nevertheless be desirable to further simply the fabricationprocess by reducing the number of implantation steps that are required.

In accordance with the present invention, the p-type floating islandsare configured as a series of coaxially located annular ledges. A methodof forming such floating islands in the voltage sustaining layer of asemiconductor power device may be generally described as follows. First,a terraced trench is formed in the epitaxial layer that is to form thevoltage sustaining region of the device. The terraced trench is formedfrom two or more co-axially located trenches that are etched atdifferent depths in the epitaxial layer The diameter of each individualtrench is greater than the diameter of the trenches located at greaterdepths in the epitaxial layer. Adjacent trenches meet in horizontalplanes to define annular ledges, which arise from the differential inthe diameter of the adjacent trenches. P-type dopant material isimplanted into both the annular ledges and the bottom of the deepesttrench in a single implantation step. If desired, the bottom trench maybe continued to form a bottom annular ring of dopant. The implantedmaterial is diffused into the portion of the voltage sustaining regionlocated immediately adjacent to and below the ledges and trench bottom.The implanted material thus forms a series of floating islands that areconfigured as coaxially-located annular rings. Finally, the trenches arefilled with a material that does not adversely affect thecharacteristics of the device. Exemplary materials that may be used forthe material filling the trenches include highly resistive polysilicon,a dielectric such as silicon dioxide, or other materials andcombinations of materials.

The power semiconductor device of the present invention may befabricated in accordance with the following exemplary steps, which areillustrated in FIGS. 5( a)-5(f).

First, the N-type doped epitaxial layer 501 is grown on a conventionallyN+ doped substrate 502. Epitaxial layer 501 is typically 15-50 micronsin thickness for a 400-800 V device with a resistivity of 5-40 ohm-cm.Next, a dielectric masking layer is formed by covering the surface ofepitaxial layer 501 with a dielectric layer, which is thenconventionally exposed and patterned to leave a mask portion thatdefines the location of the trench 520 ₁. The trench 520 ₁ is dry etchedthrough the mask openings by reactive ion etching to an initial depththat may range from 5-15 microns. In particular, if “x” is the number ofequally spaced horizontal rows of floating islands that are desired, thetrench 520 should be initially etched to a depth of approximately1/(x+1) of the thickness of the portion of epitaxial layer 502 that isbetween the subsequently-formed bottom of the body region and the top ofthe N+ doped substrate. The sidewalls of each trench may be smoothed, ifneeded. First, a dry chemical etch may be used to remove a thin layer ofoxide (typically about 500-1000 A) from the trench sidewalls toeliminate damage caused by the reactive ion etching process. Next, asacrificial silicon dioxide layer is grown over the trench 520 ₁. Thesacrificial layer is removed either by a buffer oxide etch or an HF etchso that the resulting trench sidewalls are as smooth as possible.

In FIG. 5( b), a layer of silicon dioxide 524 ₁ is grown in trench 520₁. The thickness of the silicon dioxide layer 524 ₁ will determine thedifferential in diameter (and hence the radial width of the resultingannular ledge) between trench 520 ₁ and the trench that is to besubsequently formed. Oxide layer 524 ₁ is removed from the bottom of thetrench 520 ₁.

In FIG. 5( c), a second trench 520 ₂ is etched through the exposedbottom of the trench 520 ₁. In this embodiment of the invention thethickness of trench 520 ₂ is the same as the thickness of trench 520 ₁.That is, trench 520 ₂ is etched by an amount approximately equal to1/(x+1) of the thickness of the portion of epitaxial layer 501 that islocated between the bottom of the body region and the N+-dopedsubstrate. Accordingly, the bottom of trench 520 ₂ is located at a depthof 2/(x+1) below the bottom of the body region.

Next, in FIG. 5( d), a third trench 520 ₃ (most clearly seen in FIGS. 5(e) and 5(f)) may be formed by first growing an oxide layer 524 ₂ on thewalls of trench 520 ₂. Once again, the thickness of the silicon dioxidelayer 524 ₂ will determine the differential in diameter (and hence theradial width of the resulting annular ledge) between trench 520 ₂ andtrench 520 ₃. Oxide layer 524 ₂ is removed from the bottom of the trench520 ₂. This process can be repeated as many times as necessary to formthe desired number of trenches, which in turn dictates the number ofannular ledges that are to be formed. For example, in FIG. 5( d), fourtrenches 520 ₁-520 ₄ (more clearly seen in FIG. 5( e)) are formed.

In FIG. 5( e), the various layers of oxide material located on thesidewalls of the trenches 520 ₁-520 ₄ are removed by etching to defineannular ledges 546 ₁-546 ₃. Next, an oxide layer 540 of substantiallyuniform thickness is grown in the trenches 520 ₁-520 ₄. The thickness ofoxide layer 540 should be sufficient to prevent implanted atoms frompenetrating through the sidewalls of the trenches into the adjacentsilicon, while allowing the implanted atoms to penetrate through theportion of oxide layer 540 located on the ledges 546 ₁-546 ₃ and thetrench bottom 555.

The diameter of trenches 520 ₁-520 ₄ should be selected so that theresulting annular ledges 546 ₁-546 ₃ and the trench bottom all have thesame surface area. In this way, when a dopant is introduced into theledges and trench bottom, each resulting horizontal plane of floatingislands will have the same total charge.

Next, in FIG. 5( f), a dopant such as boron is implanted through theportion of oxide layer 540 located on the ledges 546 ₁-546 ₃ and thetrench bottom 555. The total dose of dopant and the implant energyshould be chosen such that the amount of dopant left in the epitaxiallayer 501 after the subsequent diffusion step is performed satisfies thebreakdown requirements of the resulting device. A high temperaturediffusion step is performed to “drive-in” the implanted dopant bothvertically and laterally, thus defining the coaxially located floatingislands 550 ₁-550 ₄.

The terraced trench, which is composed of individual trenches 520 ₁-520₄, is next filled with a material that does not adversely affect thecharacteristics of the device. Exemplary materials include, but are notlimited to, thermally grown silicon dioxide, a deposited dielectric suchas silicon dioxide, silicon nitride, or a combination of thermally grownand deposited layers of these or other materials. Finally, the surfaceof the structure is planarized as shown in FIG. 5( f). FIG. 5( g) showsthe structure of FIG. 5( f), but with the bottom trench etched furtherto form a bottom annular ring of dopant.

The aforementioned sequence of processing steps resulting in thestructure depicted in FIG. 5( f) and 5(g) provide a voltage sustaininglayer with a series of annular floating islands on which any of a numberof different power semiconductor devices can be fabricated. Aspreviously mentioned, such power semiconductor devices include verticalDMOS, V-groove DMOS, and trench DMOS MOSFETs, IGBTs and other MOS-gateddevices. For instance, FIG. 4 shows an example of a MOSFET that may beformed on the voltage sustaining region of FIG. 5. It should be notedthat while FIG. 5 shows a single terraced trench, the present inventionencompasses a voltage sustaining regions having single or multipleterraced trenches to form any number of columns of annular floatingislands.

Once the voltage sustaining region and the floating islands have beenformed as shown in FIG. 5, the MOSFET shown in FIG. 4 can be completedin the following manner. The gate oxide is grown after an active regionmask is formed. Next, a layer of polycrystalline silicon is deposited,doped, and oxidized. The polysilcon layer is then masked to form thegate regions. The p+ doped deep body regions 405 b are formed usingconventional masking, implantation and diffusion steps. For example, thep+-doped deep body regions are boron implanted at 20 to 200 KeV with adosage from about 1×10¹⁴ to 5×10¹⁵/cm². The shallow body region 405 a isformed in a similar fashion. The implant dose for this region will be1×10¹³ to 5×10¹⁴/cm² at an energy of 20 to 100 KeV.

Next, a photoresist masking process is used to form a patterned maskinglayer that defines source regions 407. Source regions 407 are thenformed by an implantation and diffusion process. For example, the sourceregions may be implanted with arsenic at 20 to 100 KeV to aconcentration that is typically in the range of 2×10¹⁵ to 1.2×10¹⁶/cm².After implantation, the arsenic is diffused to a depth of approximately0.5 to 2.0 microns. The depth of the body region typically ranges fromabout 1-3 microns, with the P+ doped deep body region (if present) beingslightly deeper. Finally, the masking layer is removed in a conventionalmanner. The DMOS transistor is completed in a conventional manner byetching the oxide layer to form contact openings on the front surface. Ametallization layer is also deposited and masked to define thesource-body and gate electrodes. Also, a pad mask is used to define padcontacts. Finally, a drain contact layer is formed on the bottom surfaceof the substrate.

It should be noted that while a specific process sequence forfabricating the power MOSFET is disclosed, other process sequences maybe used while remaining within the scope of this invention. Forinstance, the deep p+ doped body region may be formed before the gateregion is defined. It is also possible to form the deep p+ doped bodyregion prior to forming the trenches. In some DMOS structures, the P+doped deep body region may be shallower than the P-doped body region, orin some cases, there may not even be a P+ doped deep body region.

Although various embodiments are specifically illustrated and describedherein, it will be appreciated that modifications and variations of thepresent invention are covered by the above teachings and are within thepurview of the appended claims without departing from the spirit andintended scope of the invention. For example, a power semiconductordevice in accordance with the present invention may be provided in whichthe conductivities of the various semiconductor regions are reversedfrom those described herein. Moreover, while a vertical DMOS transistorhas been used to illustrate exemplary steps required to fabricate adevice in accordance with the present invention, other DMOS FETs andother power semiconductor devices such as diodes, bipolar transistors,power JFETs, IGBTs, MCTs, and other MOS-gated power devices may also befabricated following these teachings.

1. A power semiconductor device made in accordance with a methodcomprising the steps of: A. providing a substrate of a firstconductivity type; B. forming a voltage sustaining region on saidsubstrate;
 1. depositing an epitaxial layer on the substrate, saidepitaxial layer having a first conductivity type;
 2. forming at leastone terraced trench in said epitaxial layer, said terraced trench havinga plurality of portions that differ in width to define at least oneannular ledge therebetween;
 3. depositing a barrier material along thewalls and bottom of said trench;
 4. implanting a dopant of a secondconductivity type through the barrier material lining said at least oneannular ledge and said trench bottom and into adjacent portions of theepitaxial layer;
 5. diffusing said dopant to form at least one annulardoped region in said epitaxial layer and at least one other regionlocated below said annular doped region in said epitaxial layer; 6.depositing a filler material in said terraced trench to substantiallyfill said terraced trench; and C. forming over said voltage sustainingregion at least one region of said second conductivity type to define ajunction therebetween.
 2. A power semiconductor device made inaccordance with a method comprising the steps of: A. providing asubstrate of a first conductivity type; B. forming a voltage sustainingregion on said substrate by:
 1. depositing an epitaxial layer on thesubstrate, said epitaxial layer having a first conductivity type; 3.forming at least one terraced trench in said epitaxial layer, saidterraced trench having a plurality of portions that differ in width todefine at least one annular ledge therebetween;
 3. depositing a barriermaterial along the walls and bottom of said trench;
 4. implanting adopant of a second conductivity type through the barrier material liningsaid at least one annular ledge and said trench bottom and into adjacentportions of the epitaxial layer;
 5. diffusing said dopant to form atleast one annular doped region in said epitaxial layer and at least oneother region located below said annular doped region in said epitaxiallayer;
 7. depositing a filler material in said terraced trench tosubstantially fill said terraced trench; and C. forming over saidvoltage sustaining region at least one region of said secondconductivity type to define a junction therebetween, wherein saidplurality of portions of the terraced trench are coaxially located withrespect to one another, and wherein said plurality of portions of theterraced trench includes at least three portions that differ in widthfrom one another to define at least two annular ledges and said at leastone annular doped region includes at least two annular doped regions,and wherein the step of forming at least one terraced trench includesthe steps of successively etching said at least three portions of theterraced trench beginning with a largest width portion and ending with asmallest width portion.
 3. A power semiconductor device made inaccordance with a method comprising the steps of: A. providing asubstrate of a first conductivity type; B. forming a voltage sustainingregion on said substrate by:
 1. depositing an epitaxial layer on thesubstrate, said epitaxial layer having a first conductivity type; 4.forming at least one terraced trench in said epitaxial layer, saidterraced trench having a plurality of portions that differ in width todefine at least one annular ledge therebetween;
 3. depositing a barriermaterial along the walls and bottom of said trench;
 4. implanting adopant of a second conductivity type through the barrier material liningsaid at least one annular ledge and said trench bottom and into adjacentportions of the epitaxial layer;
 5. diffusing said dopant to form atleast one annular doped region in said epitaxial layer and at least oneother region located below said annular doped region in said epitaxiallayer;
 8. depositing a filler material in said terraced trench tosubstantially fill said terraced trench; and C. forming over saidvoltage sustaining region at least one region of said secondconductivity type to define a junction therebetween, wherein step (C)further includes the steps of: forming a gate conductor above a gatedielectric region; forming first and second body regions in theepitaxial layer to define a drift region therebetween, said body regionshaving a second conductivity type; forming first and second sourceregions of the first conductivity type in the first and second bodyregions, respectively.
 4. A power semiconductor device comprising: asubstrate of a first conductivity type; a voltage sustaining regiondisposed on said substrate, said voltage sustaining region including: anepitaxial layer having a first conductivity type; at least one terracedtrench located in said epitaxial layer, said terraced trench having aplurality of regions that differ in width to define at least one annularledge therebetween; at least one annular doped region having a dopant ofa second conductivity type, said annular doped legion being located insaid epitaxial layer below and adjacent to said annular ledge; a fillermaterial substantially filling said terraced trench; and at least oneactive region of said second conductivity disposed over said voltagesustaining region to define a junction therebetween.
 5. The device ofclaim 4 wherein said plurality of portions of the terraced trenchincludes a smallest width portion and a largest width portion, saidsmallest width portion being located at a depth in said epitaxial layersuch that it is closer to the substrate than a largest width portion. 6.The device of claim 5 wherein said plurality of portions of the terracedtrench are coaxially located with respect to one another.
 7. The deviceof claim 4 wherein said plurality of portions of the terraced trenchincludes at least three portions that differ in width from one anotherto define at least two annular ledges and said at least one annulardoped region includes at least two annular doped regions.
 8. The deviceof claim 6 wherein said plurality of portions of the terraced trenchincludes at least three portions that differ in width from one anotherto define at least two annular ledges and said at least one annulardoped region includes at least two annular doped regions.
 9. The deviceof claim 4 wherein said epitaxial layer has a given thickness andfurther comprising the step of etching a first portion of the terracedtrench by an amount substantially equal to 1/(x+1) of said giventhickness where x is equal to or greater than a prescribed number ofannular doped regions to be formed in the voltage sustaining region. 10.The device of claim 4 wherein said material filling the trench is adielectric material.
 11. The device of claim 10 wherein said dielectricmaterial is silicon dioxide.
 12. The device of claim 11 wherein saiddielectric material is silicon nitride.
 13. The device of claim 4wherein said dopant is boron.
 14. The device of claim 8 wherein asurface area of the at least two annular ledges are substantially equalto one another.
 15. The device of claim 4 wherein said at least oneactive region further includes: a gate dielectric and a gate conductordisposed above said gate dielectric; first and second body regionslocated in the epitaxial layer to define a drift region therebetween,said body regions having a second conductivity type; and first andsecond source regions of the first conductivity type located in thefirst and second body regions, respectively.
 16. The device of claim 15wherein said body regions include deep body regions.
 17. The device ofclaim 4 wherein said terraced trench has a circular cross-section. 18.The device of claim 4 wherein said terraced trench has a cross-sectionalshape selected from the group consisting of a square, rectangle,octagon, and a hexagon.